I'd love to see how you dig the hole. If I was doing multiple poles, I'd rent an auger...otherwise it's a long spade and post holer (which doesn't sound fun at all).
It was recommended to me that a 4X4 post would need a 12" diameter hole. 6X6 would need 16"-18".
Or so I was told. I wimped out with the digging and went to adding posts to my deck instead.